Mielinização, desmielinização e remielinização no sistema nervoso central

Abstract

The myelin sheaths that surround axons in the CNS are made and maintained by oligodendrocytes. These glial cells can form variable numbers of myelin segments (internodules): from 1 to 200 so that when one oligodendrocyte is destroyed with preservation of the axon, many internodules can be lost, constituting a demyelinating process. As a consequence of the destruction of myelin and sheath cells a rapid and abundant cell response takes place. The response is made up by resident (microglia) and haematogenous phagocytes which phagocytose myelin and cellular debris leaving the axons demyelinated. Demyelinated axons may either stay demyelinated and clumped together or they may be separated by astrocytic processes, yet they can be remyelinated. The occurrence of remyelination depends upon the intensity and time of exposition to the demyelinating agent. Remyelination in the CNS with complete restoration of conduction may be made by oligodendrocytes or Schwann cells which invade the CNS when astrocytes are destroyed.

Ethidium bromide-induced demyelination of the sciatic nerve of adult Wistar rats

“…The type of relationship between Schwann cells and the naked axons was similar in all lesions regardless of the number of injections and was the same as seen during myelination (14), with the axon governing the approach and commitment of the myelinating cell. The excess of myelin secretion seen as redundant loops in most internodes has been observed in natural processes such as metamorphosis (15) and in many experimental models of demyelination where the Schwann cell is the main myelinating cell (1).…”

Behaviour of oligodendrocytes and Schwann cells in an experimental model of toxic demyelination of the central nervous system

“…The loss of astrocytes determined a breach in the glial limiting membrane (GLM) that allowed the free entry of Schwann cells within the CNS territory 2,7,8 , feature reported elsewhere 15,16 .The bulk of the invading Schwann cells is larger than in most toxic demyelinating models where the target of the chemical is the myelin sheath itself 14 . In the spinal cord the number of invading Schwann cells was much more marked than in the brainstem 8,18 .…”

“…Oligodendrocytes and Schwann cells perform the unique task of producing and maintaining myelin sheaths around selected axons respectively in the central nervous system (CNS) and in the peripheral nervous system (PNS) 1,2 . Although both cells have been exhaustively investigated either in normality or in disease 1,3 , many questions remain unanswered concerning the proliferating capacity of oligodendro-cytes within demyelinating lesions [3][4][5] and the origin of those Schwann cells that invade the CNS to repair the lost myelin sheaths [6][7][8] .…”

Função E Importância Da Mielina No Sistema Nervoso Central: Mielinização E Desmielinização

“…A linha densa principal (LDP) é formada pelas superfícies citoplasmáticas das membranas dos processos mielinizantes colocadas em justaposição. Os dois folhetos exteriores fundidos (aposição extracelular) formam as linhas intraperiódicas (LIP), menos eletrodensas (GRAÇA, 1988;BONDAN;LALLO, 1998). A periodicidade das lamelas, ou seja, a distância entre os centros das LDP, é diferente na mielina central e periférica, sendo de 10,7 ( 5) nm para a primeira e de 11,9 ( 7) nm para a segunda, variação que possibilita a distinção das bainhas no SNC e do SNP em cortes transversais.…”